The present invention relates to lubricating oil compositions which provide high performance standards particularly in stationary gas engines.
There is continuous need for improving the performance characteristics of engines, in particular stationary gas engines, and the lubricating oils used therein. Stationary gas engines are typically large, heavy duty, stationary engines designed to run on natural gas and other like fuels. Trends in such engines include the development of smaller four-cycle, lean burning engines, for which high performance lubricants are important.
There has been a great deal of research reported on various lubricant formulations to solve specific problems. For example, U.S. Pat. No. 5,259,967, Ripple, Nov. 9, 1993, discloses a lubricating oil composition providing less than 1% sulfated ash, comprising an additive package of a carboxylic dispersant, a rust inhibiting mixture, a hydrocarbyl substituted phenol, and a neutralized acid or phenol. The lubricant can be used in gasoline and diesel engines.
European Patent Publication EP 725129 A, Aug. 7, 1996, discloses a low sulfate ash lubricating oil composition comprising an oil of lubricating viscosity, a calcium overbased acidic material, a magnesium overbased material, and a combination of an alkylene-coupled hindered phenol antioxidant and another antioxidant, particularly useful for lubricating stationary gas engines.
PCT Patent Publication WO 01/74978, Oct. 11, 2001, discloses compositions suitable for use as lubricant additives which contain an ester-substituted hindered phenol antioxidant and other additives suitable for lubricants such as a detergent or a dispersant.
U.S. Pat. No. 6,147,035, Sougawa et al, Nov. 24, 2000, discloses a lubricating oil composition containing overbased metal salicylate, amine antioxidant, phenol antioxidant, polyalkenylsuccinimide, and zinc dialkyldithiophosphate. The lubricant is suitable as a long-life engine oil for gas engine heat pumps.
Many stationary gas lubricant formulations produce an undesirable amount of piston deposits during field use. It has been observed that a correlation exists between high levels of nitration of the lubricant during service, and these unacceptable piston deposits. The present invention focuses on improving piston deposits in such engines by way of reducing the extent of nitration of the lubricant.